Electron- and Hole-Mediated Reactions in UV-Irradiated O2 Adsorbed on Reduced Rutile TiO2(110)

The ultraviolet (UV) photon-stimulated reactions in oxygen adsorbed on reduced TiO 2 (110) at low temperatures (<100 K) are studied. When a single O 2 is chemisorbed in each bridging oxygen vacancy, only ∼14% of the O 2 desorbs after prolonged UV irradiation. For the remaining O 2 on the surface after irradiation, about one-half dissociates, and the other one-half is left in a nondissociated state that is inactive for hole-mediated photodesorption. For the maximum coverage of chemisorbed oxygen, the fraction of O 2 that photodesorbs increases substantially, but is still only ∼40%. However, when physisorbed oxygen is also present, ∼70% of the initially chemisorbed O 2 photodesorbs. On the basis of the experimental results, we propose that both hole- and electron-mediated reactions with O 2 chemisorbed on TiO 2 (110) are important. Hole-mediated reactions lead to O 2 photodesorption, while electron-mediated reactions lead to O 2 dissociation. The electron-mediated reactions explain the low total photodesorption yield when no physisorbed O 2 is present. For a fixed amount of chemisorbed 18 O 2, its PSD yield increases substantially if 16 O 2 is subsequently chemisorbed, indicating that the hole-mediated O 2 photodesorption probability depends on the charge state of the chemisorbed O 2; it decreases for more negatively charged O 2 . Because the charge state of the chemisorbed O 2 depends on the total oxygen coverage, the coverage influences the photodesorption process. A simple model based on the oxygen coverage and the charge of the chemisorbed oxygen, which accounts for the observations, is presented. In the model, O 2 chemisorbs as either O 2 − or O 2 2− depending on the oxygen coverage. O 2 − (O 2 2− ) reacting with a hole leads to O 2 0 desorption with a high (low) probability. O 2 2− plus an electron typically leads to O 2 dissociation, while O 2 − + e − does not lead to dissociation.